1. Field of the Invention
The invention refers to a device comprising an implant with at least two clearance holes for screws, in particular a plate with holes, the device also comprising screws for fastening said implant to a bone and having a threaded portion to be screwed into the bone, and retaining means comprising a collar protruding in a generally radial direction and intended for retaining the implant at its side facing away from the bone.
The invention also refers to a device for connecting two separated pieces of bone at the place of their separation, comprising a plate with holes, furthermore screws for fastening the plate to the bone pieces and a flexible areal element to be disposed at the place of separation.
2. Description of the Prior Art
Devices comprising plates fastened by means of screws to broken bone pieces for the purpose of immobilizing these bone pieces have long been known in the field of bone surgery. Attention is called in this connection to U.S. Pat. No. 3,552,389. The Swiss Pat. No. 566 767 also discloses plates, which may be used in particular in jaw surgery. The screws of the known device comprise threaded portions of solid cross-section and heads with hemispherical bearing surfaces. The plates are provided with slots serving as clearance holes. Some of the slots may be provided with countersunk portions adapted to admit the screw heads and designed in such a manner, that the plate is displaces in a particular direction whenever the screw in question is screwed in place.
If, for example, in the course of a surgical operation two pieces of bone of the lower jaw must be connected with one another, the plate is properly formed and then fixedly fastened to the bone pieces, so that it makes contact with the two pieces under pressure. Experience has shown, however, that in many cases a reabsorption of the bone took place at the contact surface, a few weeks after the plate had been fastened. This happened in particular if time variable loads acting on the bone pieces, as tend to occur in the process of chewing, were large enough to cause sections of the plate to perform micromovements i.e. small movements relative to the bone pieces, in spite of the static compression force generated by the fastening process. Such reabsorption may cause open gaps to form between the plate and the bone pieces. The plate may then move back and forth between the screw head and the bone pieces in the longitudinal direction of the axis of the screw. Under certain circumstances this may cause the threaded portions of the screws screwed into the bone pieces to become loose too. The connection between the plate and the bone parts continues to get more and more loose, so that the plate stops fulfilling its function of fastening the bone pieces to one another.
Jaw tumors make it sometimes necessary to remove one piece of the jaw bone. In such a case too, the remaining bone pieces separated from one another by an open space are connected by way of a plate fastened by means of a screw connection. A sleeve possessing a certain stability of shape is then made of a plastically deformable wire lattice and placed over the two ends of the bone pieces facing one another. The space between the bone pieces bounded by the sleeve allows the insertion of bone material from other bones of the patient, so that the two bone pieces grow back together.
When surgically treating tumors of this kind, the bone pieces to be connected are not in contact with one another and are held together in the initial period after the operation by way of the screw-connected plate. In such cases it is of particular disadvantage to have the connection between the plate and the two pieces of bone become loose. On top of this, the sleeve surrounds the bone pieces in a loose way only.
An additional disadvantage of the known devices which shows itself in particular with plates comprising slot-shaped clearance holes, consists in that the micromovements taking place between the plate and the screws may cause frictional corrosion.
Mention is made here of the fact, that many different screw locking devices are known in the art. The Swiss Pat. No. 106 842, for example, discloses a screw locking device having a purpose not described in detail, but not intended for fastening implants. The screw icludes a slot with a wedge inserted into it. A wood screw is disclosed, among other things, provided with a conical screw head and a slot passing through one portion of the screw shaft. A metal wedge is driven into the slot and into the material of the screw-fastened portion surrounding the slot. This wedge is formed by a small plate essentially plane and provided at its forward edge with a wedge-shaped knife edge.
The Swiss Pat. No. 106 842 gives no indication to the effect that the screw locking devices disclosed therein are intended to be used for fastening implants. The screw locking device which comprises a wedge protruding sideways beyond the screw, as disclosed in Swiss Pat. No. 106 842, would in fact be unsuited for fastening an implant, because the wedge would have to be driven into the bone plate and possibly into the bone too. However, the wedge could only be driven into the bone plate, made for example of steel, if the plate had been initially provided with slots for receiving the wedge. However, slots of this kind would cause considerable weakening of the plate and, in addition, it would allow applying the wedge in certain definite positions of rotation of the screw only.
The Swiss Pat. No. 569 202 too, discloses a screw looking device comprising a screw with a head and a threaded portion, the latter being provided with slots at its end. The screw possesses a longitudinal opening with a conical portion located in the region of the slots and a threaded portion extending in continuation of the conical portion. An expander having a conical head a thread and a thinner shaft is screwed into the longitudinal opening. The Swiss Pat. No. 567 202 too, fails to give any indication relative to the possibility of using the screw locking device disclosed therein for surgical purposes. Since the device according to Swiss Pat. No. 569 202 is subject to expansion at the free end of the threaded portion of the screw, this device too, would offer no improvement in the connection between the screw and the bone plate.
The French Patent disclosure publication No. 2 193 161 refers to the riveting or the screw-fastening of sheet metal plates. A bushing provided with a conical inner surface is first inserted in each of the bores of the plates to be connected with one another. In one illustrated embodiment, a screw having a conical neck is then inserted in a manner to produce locking, subsequent to tightening the screw. Mention is made in this connection of the fact, that the bushing could be provided with a slot extending over its entire length. The French Offenlegungsschrift No. 2 193 161 this refers to the screw-fastening of sheet metal plates and does not contain anything that would indicate that the connection disclosed therein could be adapted to the fastening of bone plates.